Frequently it is desired to differentiate between odd and even fields of a video signal. This is particularly true in systems which generate composite pictures including a small auxiliary image insert within the confines of a larger main image, wherein the two images are unrelated or asynchronous, a so-called picture-in-picture of Pix-in-Pix system. In this type of system odd/even field information of both the main and auxiliary signals is utilized to maintain proper interlacing of the auxiliary signal.
Typical odd/even field detectors, such as are included in the commercially available LM 1881 Video Sync Separator available from National Semiconductor Corporation, integrate the composite synchronizing signal, and sample the integrated signal at predetermined intervals to determine whether the current field is odd or even. Other systems such as is described in U.S. Pat. No. 4,876,598 utilize monostable multivibrators to determine the relative timing of the vertical equalization pulses with respect to the horizontal synchronizing pulses of composite video signal to determine odd and even fields.
The foregoing systems utilize analog signal processing techniques which tend to be adversely affected by parametric changes due to aging, temperature variations, etc. In addition, for example in Pix-in-Pix systems, where it is desired to incorporate the majority of the signal processing of the, e.g., Pix-in-Pix function, on a single integrated circuit in digital form, it is inconvenient to incorporate analog type odd/even field detectors.